This invention relates in general to solvents for use in solubilizing dyes and, more particularly, to solvents used in the production of carbonless paper, microcapsules containing a chromogenic compound solubilized by such solvents, and pressure sensitive marking and recording material coated with such microcapsules. The invention also relates to a process for making carbonless paper solvents.
Carbonless paper and other marking systems depend upon localized contact between a chromogenic compound such as a leuco dye and a color developing substance to produce a visible indicia. In such marking systems, the dye is solubilized in a solvent and the dye and solvent are then emulsified in an aqueous solution to form tiny "oil" droplets. The oil droplets are then encapsulated by the polymerization or coagulation of a suitable material to form a wall around the individual droplets.
In one application, the microcapsules which are formed can be coated on the back side of a sheet of paper or other material to form a coated back sheet. A second sheet of paper is then coated on the front side with color developing substance, typically a proton donating component such an acidic ,clay or phenolic resin. The microcapsule outer walls isolate the dye from the developer until the microcapsules are ruptured by the application of localized pressure. When pressure is applied by a stylus, pen, typewriter, printer or similar instrument, the microcapsules burst and the oil solution containing the solubilized dye is released and is transferred to the second sheet of paper where the dye reacts with the acid component to form an image. Carbonless paper systems of this type are described in U.S. Pat. Nos. 3,418,656 and 3,418,250, which are incorporated by reference herein in their entirety.
Solvents which are utilized in carbonless paper systems must typically be substantially colorless, have a low vapor pressure, be substantially odorless, exhibit acceptable solvency, have a suitably low freezing point, and be characterized by a low viscosity. Various types of solvents can be used in such systems and their suitability will be dependent upon the characteristics of the particular dyes and developers utilized. For example, a low viscosity solvent is generally required when phenolic acid developers are utilized in order to achieve rapid image development. By contrast, when using an acid clay developer, a higher viscosity solvent can be used with acceptable results.
Many suitable carbonless paper solvents are known. For example, certain alkyl naphthalenes are disclosed as suitable carbonless paper solvents in U.S. Pat. Nos. 3,806,463 and 4,003,589 to Konishi et al. The specific solvents are represented by the general formula: EQU Ar--R.sub.n
wherein Ar is a naphthalene nucleus, R represents a C.sub.1 -C.sub.4 alkyl group, n is an integer of 1-4, and R may be the same or different when n is 2-4, with the proviso that the total number of carbon atoms in R must be 4-6. Examples of suitable solvents listed in that patent include butylnaphthalene, dimethylpropylnaphthalene, methylbutylnaphthalene, ethylpropylnaphthalene, methylpropylnaphthalene, diethylnaphthalene, dimethylbutylnaphthalene, dimethylethylnaphthalene, and dipropylnaphthalene, trimethylpropylnaphthalene. Konishi et al. specifically teach that when the total number of carbon atoms in the substituted alkyl groups exceeds 6, the odor will be less observable but problems in microcapsulation and unsatisfactory dye solubility will result.
In an article published in the Bulletin of Aichi Environmental Research Center, No. 4, pages 114-117 (1976), Kodama reported the results of the compositional analysis of a commercial carbonless paper solvent containing alkyl naphthalenes. Among the compounds specifically identified as being present in the commercial solvent were propylnaphthalene, methylpropylnaphthalene, dimethylpropylnaphthalene, dipropylnaphthalene, methyldipropylnaphthalene, dipropylisopropenylnaphthalene, and tripropylnaphthalene. Kodama did not disclose whether the propyl moieties were n-propyl or isopropyl. Many of the alkyl naphthalenes, including methyldipropylnaphthalene, were present only in minor proportions in relation to the overall mass of the solvent mixture which was analyzed.
The use of diisopropylmethylnaphthalene as one component of a rubber processing oil having good light discoloration resistance was reported in Chem. Ab. 86(6): 30825r.